Purge plug

ABSTRACT

A purge plug adapted to be replaceably attached to the bottom of a lable for purging gas through a heat in the ladle. The purge plug comprises a ceramic body having at least one through gap which extends between the end faces of the body and a casing which encloses the body. The gas is fed to the external end face of the body in order to flow, at a predetermined pressure, through said gap to the internal end face of the body and into the heat. The ceramic body comprises an external, essentially sleeve-shaped body portion and an internal, truncated cone-shaped body portion. Between them said gap is arranged. The internal body portion is movable in relation to the external body portion and is pretensioned by a resilient means in a direction towards the heat to close the gap. A gas supply pipe is connected to a space beneath the external end face to open the gap.

The present invention relates to a purge plug which is adapted to bereplaceably attached to the bottom of a ladle for purging gas through aheat in the ladle, the purge plug comprising a ceramic body having atleast one through gap which extends between the end faces of the bodyand a casing which encloses the body, the gas being fed to the externalend face of the body in order to flow, at a predetermined pressure,through said gap to the internal end face of the body and into the heat.

Purge plugs are generally used for stirring a heat in a ladle, and,where appropriate, also for modifying the heat, and serve to let gasunder high pressure, for instance 6-10 bar, into the ladle. A purge plugtraditionally consists of a gas-permeable, ceramic cone having the shapeof a truncated cone of about 10 kg which is enclosed by a sheet-metalcasing. The purge plug is generally supplied in so-called sets, that isthe cone is mounted in a hollow block which weighs about 50 kg.

The purge plug has a key role when manufacturing pure steel and is usedby practically all steel plants in the world. One or more purge plugsare replaceably mounted in the bottom of a unit which is found in allsteel plants and is called a ladle, that is a container lined withrefractory material and adapted to contain melted steel. In the ladle,various high-temperature chemical processes are carried out in which thepurge plug is crucial. Gas, generally argon, is purged through the purgeplug. The purge plug is worn heavily during the process in its uppermostportion, which is directed towards the heat, and regularly has to bereplaced by a new purge plug when its height has shrunk to a minimumpermissible level.

The first purge plugs which were made were porous, that is they weregas-permeable but did not let steel through. The disadvantage of thesepurge plugs was that high pressure gas was required to obtain asufficient flow, and owing to their porosity the wear was considerable.

The next step in the development of purge plugs was so-calleddirectional porosity, that is a number of ducts with a diameter of about0.5 mm were formed by casting and gas was purged through the ducts. Theadvantage of this purge plug was that it was easy to achieve a gas flowthrough the plug and that the plug could be made compact, which meantless wear. The disadvantage was a greater risk of infiltration of steelin the ducts, which obstructed the purge plug.

The third step in the development of purge plugs was to form slots orgaps by casting, having a thickness of about 0.2 mm and a length ofabout 20 mm, through the purge plug instead of ducts. The advantage ofthis was that it was easy to achieve a high gas flow, withoutinfiltration of steel in the slots. However, there is always a risk ofinfiltration of steel as long as there are open gaps in the purge plug.

A purge plug must fulfil three requirements: a high degree ofdurability, satisfactory gas permeability and avoidance of infiltrationof steel in the gaps of the purge plug.

Satisfactory durability is obtained by the purge plug being made of ahigh-quality, compact refractory material.

A high degree of gas permeability is obtained by the gaps having asufficient sectional area.

Minimum infiltration is obtained by the gaps being sufficiently narrow.

These three requirements counteract one another and a purge plug is acompromise of these three properties.

German patent specification DE 196 10578 discloses a purge plug whichcomprises an external body having a truncated cone-shaped cavity in itsupper portion, a cylindrical, enlarged cavity in an intermediate portionand a cylindrical duct in its lower portion. The cavities communicatewith one another to allow gas with a specific pressure flow from one endface to another. In the cavities, in the above-mentioned order, atruncated cone, a piston and a tubular piston rod which is provided withopenings are movably arranged as a unit. The cone is pretensioned to aclosing position by means of an external pressure spring which engageswith the free piston rod end protruding from the purge plug. When gaswith a sufficient pressure (in order to counteract the force of thespring and the metallurgical pressure) is supplied through a gas supplypipe and via the piston rod to the enlarged cavity, the piston raisesthe cone and the gap between the cone-shaped cavity and the cone isopened. Then the gas can flow round the piston and through the gap intothe heat.

The disadvantages of this construction are as follows:

The construction is very difficult to manufacture as purge plugs aregenerally cast in a refractory material. It is technically difficult tomake an intermediate, enlarged cavity (16) by casting since the templateused in casting is very difficult to remove.

The construction has complicated protruding parts (the spring suspensionbeneath the purge plug), which are inconvenient in a steelworksenvironment since they may easily break by being subjected to impacts orother rough handling.

The spring suspension is expensive to manufacture.

The purge plug is worn gradually. When it is worn down to the enlargedcavity (16), a breaking through of the plug is obtained and steel flowsthrough the bottom of the ladle with disastrous results. It is thus veryimportant that the plug not be worn down to this level. Advancedequipment is required in order to determine how far down the purge plughas been worn.

Another patent that is based on a purge plug having a movable body and afixed body, where the gas flows through an annular gap, is U.S. Pat. No.4,470,582.

What mainly makes this construction different from that in theabove-mentioned German patent is that the pressure spring has beenreplaced by a lever mechanism which is preloaded by means of a hydrauliccylinder.

The disadvantages of this construction are as follows:

Because of its complicated shape, it is very expensive to manufacture.

The gas flow is controlled by an external valve which opens and closesthe annular gap by guiding the movable body upwards and downwards,respectively. The disadvantage is that an external controlling device isrequired, which takes up space and is inconvenient in a steelworksenvironment. It can easily be damaged by external forces.

As in the above-mentioned German patent, the purge plug is worn with thesame result as indicated above.

One object of the present invention is to provide a purge plug having agap, the width of which is a function of the applied gas pressure andwhich is easy and inexpensive to manufacture.

Another object of the invention is to provide a purge plug having anadjustable gap which does not require complicated mechanisms and doesnot exhibit any protruding components.

Yet another object is to provide a reliable purge plug which can be worndown more or less completely before the breaking through takes place.

According to the invention these objects are achieved by a purge plug asmentioned by way of introduction, which is characterised in that theceramic body comprises an external essentially sleeve-shaped bodyportion and an internal, truncated cone-shaped body portion betweenwhich said gap is arranged, that the internal body portion is movable inrelation to the external body portion and is pretensioned by means of aresilient means in a direction towards the heat for closing the gap andthat a gas supply pipe is connected to a space beneath the external endface in order to open the gap.

Further developments of the invention are evident from the featuresindicated in the subclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the invention will now be described by way ofexample with reference to the accompanying drawings, in which

FIG. 1 is a schematic elevational cross-sectional view of the bottom ofa ladle with purge plugs,

FIG. 2 is an elevational cross-sectional view of a purge plug accordingto the invention having a closed gas flow gap,

FIG. 3 shows the purge plug according to FIG. 2 having an open gas flowgap,

FIG. 4, FIG. 5 and FIG. 6 illustrate various embodiments of the bottomplate of the purge plug, and

FIG. 7 shows an alternative embodiment of the purge plug according tothe invention.

Reference is first made to FIG. 1, which schematically illustrates thebottom portion of a ladle 1 for a heat 2, usually a heat of metal, suchas steel. A number of replaceable purge plugs 4 are, in conventionalmanner, releasably mounted to the bottom 3 of the ladle by means of perse known mounting means, as indicated by the reference numeral 5. A gassupply pipe 6 is connected to each purge plug 4. All this is prior-arttechnique and, thus, does not require any further explanation.

Reference is now made to FIGS. 2 and 3, in which the construction andfunction of the purge plug according to the invention are shown. Thepurge plug 4 comprises a casing 7. Since the purge plug 4 is to be usedfor a steel heat 2, the casing 7 is preferably made of steel. The casingis shown with the shape of a truncated cone, the tapered end being incontact with the heat, see FIG. 1, but it may also be inverted orcylindrical (not shown). A ceramic body 8 is enclosed by the casing 7and is preferably attached to the casing 7 along its whole length.Preferably, the body 8 is cast directly inside the casing.

The body 8 is, in its turn, divided into an external, concentric,essentially sleeve-shaped body portion 9 having a truncated cone-shapedinternal cavity and an internal, truncated cone-shaped body portion 11which completely fills up the cavity. The internal body portion 11 ismovable, that is longitudinally displaceable, in relation to theexternal body portion 9, with a gap 12 appearing between them. Whenmanufacturing the body portions 9 and 11, the internal body portion 11can be easily formed, well adapted to the external body portion 9, byplacing a plastic foil or plastic sheet which has been pre-formed to atruncated cone-shaped casing element concentrically in the casing 7 atthe moment of casting and then removing it when the refractory material(the ceramic compound) has solidified. Furthermore, in connection withthe casting, threaded blind holes 13 can be formed or threaded sleeveinserts be cast into the larger end face 14 of the internal body portion11.

Preferably, but not necessarily, the larger end face 15 of the externalbody portion 9 is on a level with the above-mentioned end face 14. Thesame goes for the smaller end faces 16 and 17, respectively, of the bodyportions 9 and 11.

The internal body portion 11 is displaceable downwards relative to thefixed external body portion 9 (see FIG. 2) and is pretensioned upwardsto close the gap 12 by means of a resilient or elastic means which actsagainst the end face 14.

In the preferred embodiment a resilient bottom plate 18 isgas-impermeably attached to the casing 7 at a distance from the externalend face of the body 8, in this case the end faces 14 and 15, a space orchamber 19, which can be pressurised, being defined between them. Thisdistance can be small and even non-existent within limited areas of theend faces 14 and 15—the only condition being that a chamber 19 is formedwhen pressurised fluid (purging gas) is supplied. A gas supply pipe 6 isconnected to the bottom plate 18 and is connected to the chamber 19 forpressurisation of the same with the purging gas 21. The internal bodyportion 11 is fixedly attached to the bottom plate 18, for instance, bymeans of bolts 22 or some other suitable attaching means. The gap 12 iskept closed by the internal body portion 11 being pressed upwards inFIG. 2 into abutment against the internal peripheral surface of theexternal body portion 9 by means of the pretensioning of the bottomplate 18 and/or the pressure of the purging gas.

When it is desirable to purge gas through the heat in the ladle, gas issupplied under high pressure to the space 19. Since the bottom surfaceor end face 15 of the external body portion 9 is larger than the bottomsurface or end face 14 of the internal body portion 11, the bottom plate18 is caused to bend outwards, see FIG. 3, and pulls along the internalbody portion 11, whereby the annular gap 12 is opened and the gas canflow into the heat. The more pressure one applies, the more the gapopens. When the gas flow is stopped or the pressure of the gas isdecreasing, the bottom plate 18 springs back and pushes the internalbody 11 back and closes the gap completely. This prevents infiltrationof the heat into the gap 12. The construction of this new purge plugmeans that it is possible to obtain an almost unlimited gas flow withouthaving any infiltration of steel into the gap in the purge plug. This isachieved by the size of the gap varying with the applied pressure.

In FIGS. 2 and 3, the bottom plate 18 is shown to have a constantthickness. However, it is of course possible to control the function ofthe pressure of the gas on the gap width by using bottom plates 18 withspecific resilience properties. Apart from using bottom plates ofvarious thicknesses for this aim, it is possible to achieve the desiredspring properties by giving the bottom plate a thickness that is noteven, that is having its thickness varying in a direction away from itscircumference towards its centre. An example of this has beenillustrated in FIG. 4. As shown in FIG. 5, it is also possible to obtainthe required resilience properties by providing a bottom plate ofuniform thickness with stiffening flanges or ribs 23 which are attachedperpendicular to the bottom plate, preferably radially oriented on thebottom plate, and exhibiting a non-constant height or thickness.Naturally, the above-mentioned features can also be combined in order toachieve the desired function, that is the width of the gap 12 atdifferent specific gas pressure.

A different way to obtain the desired resilience properties of thebottom plate, which is also compatible with those mentioned above, is tomake the bottom plate 18 curved, for instance inwards, as shown in FIG.6. For this purpose, it is also possible to make the bottom plateundulated (not shown).

In order to limit the deflection of the bottom plate, for instance atextremely high gas pressure or when the width of the gap is not allowedto exceed a certain dimension, for example 0.5 mm, it is suitable tosupply the purge plug with a deflection-limiting means, for instance oneor more girder elements 24 indicated by dashed lines in FIG. 6. Eachgirder element 24 is fixedly attached to the casing 7 (see FIG. 6)and/or is secured to the external body portion 9 (not shown). Theabove-mentioned means can, of course, also be a thick, essentially rigidplate, cf. the bottom plate of a conventional purge plug. When required,connecting elements may be used between the outermost portion of thecasing and the ends of the deflection-limiting means (not shown). When adeflection-limiting means as described above is used, it may, in certaincases, be suitable to arrange yet another resilient means between saidmeans 24 and the bottom plate 18 (not shown).

The circumferential surface of the internal body portion 11 and theinternal circumferential surface of the external body portion 9 define,according to that mentioned above, the shape of the gap 12. Usually, itis preferred that the cross-sectional profile of the gap, that is theshape of the gap in a plane perpendicular to the longitudinal axis ofthe purge plug (a plane which is parallel to the bottom plate) is acircle or an oval. However, the cross-sectional profile of the gap istotally optional, such as a triangle, square or polygon, or star-shapedsince the two body portions 9 and 11 can be cast at the same time,having between them a separating casing element of plastic foil orplastic sheet with a chosen cross-sectional profile, which, after thesolidification of the compound, can be easily removed.

In order to obtain special gas flow properties, for instance precipitatechanges in the gas flow through the purge plug, it is further possibleto form the internal body portion 11 with a truncated cone-shapedcavity, in which a correspondingly formed truncated cone-shapedinnermost body portion 25 is displaceably arranged, see FIG. 7. Thisinnermost body portion 25 works in the same way as described inconnection with the internal body portion 11 since the former is alsofixedly attached to the bottom plate 18, for example by means of a bolt26. The resilience properties of the bottom plate is accordinglyadjusted by means of the modifications described above, cf. FIGS. 4 and5. The gap 27 between the innermost body portion 25 and the internalbody portion 11 may be given an optional shape in accordance with thatdescribed above and, preferably, the gap 27 is formed in the same manneras the gap 12, that is by means of yet another casing element which isarranged concentrically with the above described casing element in thecasing 7.

It should be noted that every movable body portion 11, 25 tapers offupwards, that is from the external end face 14, 15 towards the internalend face 16, 17 adjacent to the heat in the shown embodiments of thepurge plug according to the invention.

The following properties are achieved by means of the purge plugaccording to the invention:

It can easily be manufactured (as inexpensive as the purge plugs whichare available on the market today).

Infiltration is completely avoided.

The ratio pressure/flow may be adapted to the conditions of therespective users.

An unlimited gas flow can be obtained.

The invention is not limited to that described above or that shown inthe drawings but can be changed within the scope of the appended claims.

What is claimed is:
 1. A purge plug which is adapted to be replaceablyattached to the bottom of a ladle for purging gas through a heat in theladle, the purge plug comprising: a ceramic body having at least onethrough gap which extends between an internal end face and an externalend face of the body; a casing which encloses the body; the ceramic bodycomprises an external sleeve-shaped body portion defining the throughgap, and an internal, truncated conical body portion movably disposedinside the through gap and being adapted to define a gap with walls ofthe through gap, the internal body portion being pretensioned by aresilient member in a direction towards the heat to close the gap; and agas supply pipe connected to a space beneath the external end face ofthe body through which gas is adapted to be supplied under pressure toovercome a force of the resilient member and open the gap.
 2. A purgeplug as claimed in claim 1, wherein the resilient member includes aresilient bottom plate gas-impermeably attached to the casing at adistance from the external end face of the body the space being at leastpartially defined by the casing, the external end face, and the bottomplate, the internal body portion being attached to the bottom plate. 3.A purge plug as claimed in claim 2, wherein the thickness of the bottomplate varies in a direction from its circumference towards its center.4. A purge plug as claimed in claim 2, wherein the bottom plate isprovided with radially oriented stiffening flanges.
 5. A purge plug asclaimed in claim 2, wherein the bottom plate is curved.
 6. A purge plugas claimed in claim 1, wherein a cross-sectional profile of the gap is acircle or an oval.
 7. A purge plug as claimed in claim 1, wherein across-sectional profile of the gap is a triangle, square or polygon. 8.A purge plug as claimed in claim 2, wherein means for limitingdeflection of the bottom plate is arranged outside the space at adistance from the bottom plate and is attached to at least one of theexternal body portion and the casing.
 9. A purge plug as claimed inclaim 8, wherein the limiting means is a girder element having ends ofwhich are fixedly attached to the casing.
 10. A purge plug as claimed inclaim 8, wherein additional resilient member is arranged between thebottom plate and the limiting means.
 11. A purge plug as claimed inclaim 3, wherein the bottom plate is provided with radially orientedstiffening flanges.
 12. A purge plug as claimed in claim 3, wherein thebottom plate is curved.
 13. A purge plug as claimed in claim 4, whereinthe bottom plate is curved.
 14. A purge plug as claimed in claim 2,wherein a cross-sectional profile of the gap is a circle or an oval. 15.A purge plug as claimed in claim 3, wherein a cross-sectional profile ofthe gap is a circle or an oval.
 16. A purge plug as claimed in claim 4,wherein a cross-sectional profile of the gap is a circle or an oval. 17.A purge plug as claimed in claim 5, wherein a cross-sectional profile ofthe gap is a circle or an oval.
 18. A purge plug as claimed in claim 2,wherein a cross-sectional profile of the gap is a triangle, square orpolygon.
 19. A purge plug as claimed in claim 3, wherein means forlimiting deflection of the bottom plate is arranged outside the space ata distance from the bottom plate and is attached to at least one of theexternal body portion and the casing.
 20. A purge plug as claimed inclaim 9, wherein additional resilient member is arranged between thebottom plate and the limiting means.
 21. A purge plug as claimed inclaim 1, wherein the resilient member includes resilient means.